As progress has been made in engineering technologies such as vehicles and electrical equipment over the years, many types of equipment have become smaller, lighter and higher in output, and operating conditions have increased in temperature and become harsher. As a result, greases used in a variety of equipment have needed to exhibit improved performance at higher temperatures, and grease compositions having high dropping points and excellent thermal stability have been proposed.
Meanwhile, recent years have seen an increase in demand not only for improved performance of greases at high temperatures, but also for human safety during use and the use of materials having little environmental impact during production, and a grease that fulfils these requirements is needed. Of these requirements, grease compositions that contain lithium complex soaps, which are obtained by improving lithium soaps, or urea as thickening agents have high dropping points and exhibit excellent heat resistance. Therefore, various proposals have been made for these types of grease composition in order to further improve these properties.
As a grease composition that contains a lithium soap as a thickening agent, JP 2006-131721 proposes a lithium complex grease which comprises a lithium salt of an aliphatic monocarboxylic acid, a lithium salt of an aromatic dibasic acid and a lithium salt of an aliphatic dibasic acid, which has a higher dropping point than a lithium grease and which has a wide range of usage temperatures. However, lithium, which is a raw material of the lithium grease, is used in a wide variety of applications in addition to greases, and because there has been a high demand for lithium recently, there are concerns that lithium resources will become depleted and the price of lithium will increase in the future. In addition, because the lithium complex grease involves reacting two types of fatty acid in two stages, the production process of the lithium complex grease is complicated and requires a long period of time.
In addition, as a grease composition that uses urea as a thickening agent, JP 2008-231310 proposes a diurea grease able to be used at high temperatures for a long period of time. However, amine compounds such as aniline, which are used as raw materials, are extremely toxic and must be handled with sufficient care during production, meaning that safety is an issue.
As a result, grease compositions that use calcium as a thickening agent, which are superior in terms of safety, environmental burden and production costs, have been investigated as replacements for grease compositions that use lithium soaps or urea as thickening agents, and cannot therefore be said to be satisfactory in terms of safety and environmental burden.
However, greases that use calcium soaps as thickening agents are generally inferior to lithium greases, lithium complex greases and urea greases in terms of dropping point and heat resistance, and do not therefore fulfil the recent requirements of greases.
Proposals have been made for greases known as calcium complex greases, which generally use a calcium complex soap of a higher fatty acid and a lower fatty acid as a thickening agent, as greases that fulfil such requirements.
In particular, JP 2009-249419 proposes a calcium complex grease, which uses calcium salts of a dibasic acid and a fatty acid as a thickening agent, as a calcium complex grease having a high dropping point. However, in addition to the problem of being unable to maintain a suitable thickness if the added quantity of the thickening agent is low, this calcium complex grease is limited in terms of the form of the dibasic acid, and especially terephthalic acid, used as a raw material, and involves production problems such as requiring terephthalic acid to be introduced at the high temperature of 120° C.
The problem to be addressed by the present invention is to provide a calcium complex grease which exhibits equivalent or superior heat resistance to a grease that uses a lithium soap or urea as a thickening agent by maintaining (having or ensuring) a high dropping point and which can maintain a suitable thickness even if the quantity of thickening agent is low.